1. Field of the Invention
The present invention relates to a tread band for tires which improves the comfort and handling characteristics of the tire, and more particularly to a tread band for improving the handling and comfort characteristics of a tire with an asymmetrical tread design.
2. Description of Related Art
Tires provide the connection between a vehicle and the road upon which the vehicle travels, and are designed to transfer to the road as much as possible, the acceleration, braking, and directional forces that are generated by the vehicle. These forces act on the road by the friction existing between the elastomeric compound of the tire and the rough surface of the road. To obtain good handling characteristics, the friction between the tire and the road must be maximized by a well-designed tire under a variety of driving conditions, for example when operating over dry, wet, snowy and icy surfaces. In addition, a tire should also provide other functional features, such as a comfortable and smooth ride, and long mileage life. These characteristics should also be maintained when the tire is used under unusual operating conditions, such as when the tire is driven at excessive speeds, over less than ideal road surfaces, or when the ambient temperature reaches high or low extremes.
The performance properties of the tire depend on several characteristics of the tire construction. One of the mechanical characteristics of a tire is the tread pattern provided on its tread band. The tread pattern consists of a series of grooves criss-crossing the tread band both circumferentially and transversely, defining discrete solid blocks and ribs in the tread band. These blocks and ribs are arranged to form a specific pattern. This pattern is varied depending on the expected use of the tire and, for example, may be optimized for handling on dry roads, on wet roads, or in snow and icy conditions.
The tread pattern may be characterized by its net to gross ratio, which is a measure of what proportion of the surface of the tread is actually in contact with the ground. The net to gross ratio is derived by dividing the area of the blocks between the tread edges in actual contact with the ground (the net contact area) by the total surface of blocks and grooves between the tread edges (the gross area). The greater the net to gross ratio, the more rubber is in contact with the ground, for similar tires of equal tread width.
Several general types of tread patterns are commonly known and used for tires. A symmetrical tread pattern is the most common and has a design that appears the same regardless of the direction from which an observer looks at the tire. This type of tire, with a symmetrical tread band, can be mounted on either side of the vehicle and is suitable for rotation in either direction. Asymmetrical tires are also becoming more common. A tire is defined as asymmetrical when it has a tread band divided in two portions by a plane perpendicular to the tire axis of rotation. Each tread band portion has a different design, optimized for a specific purpose. For example, the tread band portion closest to the vehicle can be optimized for wet surface handling, while the other portion can be optimized for cornering. An asymmetrical tire must be mounted in a specific orientation, with one predetermined tread band portion always mounted nearest to the vehicle. Finally, tires with directional tread bands can also be used. A tread band is directional when the tread pattern is specular with respect to the equatorial plane of the tire, and the tire is designed to be mounted in a preferred directional rotation relative to the vehicle.
The tread pattern itself can also be optimized for different operating requirements. For example, the tread pattern can be designed to better resist wear and to provide better handling in dry conditions, or can be designed to be more resistant to hydroplaning and to provide better traction on wet or snowy surfaces. The design of the tread pattern also has an effect on the comfort and smoothness of the ride of the tire. The tread pattern can help maintain directional stability of the tire, and promote smooth and quiet rolling contact of the tire with the road.
It is a difficult task to combine good handling capability with good resistance to wear of the tire, both in the presence of normal tire operating temperatures, which range between 30xc2x0 C. and 70xc2x0 C., and high operating temperatures, which can be more than 70xc2x0 C., and may even surpass 100xc2x0 C. in some cases. High tire operating temperatures are caused by the thermo-mechanical stresses produced within the tire, especially when driving at high speed and on curvy roads, and tend to reduce the life of the tire. At low temperatures, the elastomeric compounds of the tire tend to wear less, but also provide less friction with the road and, thus, reduced handling performance. At higher temperatures, performances are increased but resistance to wear is degraded.
It is also necessary for the tire to provide the vehicle with a smooth and comfortable ride. Tires that are optimized for good handling tend to be made from stiffer compounds, which often give a harsher and less comfortable ride than softer, more pliable compounds. This can be a problem especially on long trips, because a harsh ride can contribute to driver fatigue.
The conflicting requirements for tire designs of simultaneously providing good handling capabilities, a comfortable and smooth ride, good wear resistance, and low rolling resistance have been difficult to achieve. This has resulted in various compromises in the design of conventional tires. Compromises are made both in the materials used to construct the tire, especially the tread band of the tire, and also in the tread pattern that is formed on the tread band of the tire. The desire to provide good handling properties in all types of weather conditions and temperatures, and to obtain a smooth and comfortable ride have thus required less than ideal selections in both the design of the tread pattern and in the compounds used to form the tread band. Conventionally, attempts have been made to combine different elastomeric compounds in the construction of a tread band to meet the many demands imposed by conflicting requirements for tire performance. For example, British patent 1,588,575 describes a motor vehicle tire having treads capable of providing good traction on roads covered with ice and snow, as well as dry roads. The tire tread of this patent has two separate annular circumferentially extending portions contiguous with each other, and having different configurations from one another, with each made of an elastomeric composition having a second order temperature (also known as glass transition temperature) different from that of the composition forming the other portion.
Another example of a method for reducing the working temperature of a tire tread for vehicles was described in European Patent Application Number EP 864446. In that application, a tire tread was used consisting of a first portion A comprising 100 parts by weight of an elastomeric material, 40-120 parts by weight of a filler comprising from 50 to 100% by weight of carbon black and from 0 to 50% of silica, and 3-40 parts by weight of at least a conventional additive. The tread also had a second portion B comprising 100 parts by weight of an elastomeric material, 40-120 parts by weight of a filler comprising from 30 to 100% by weight of silica and from 0 to 70% of carbon black, and 3-40 parts by weight of at least a conventional additive. In all cases, the silica content in portion B was at least 20% higher than in portion A.
The Applicant realized that if asymmetrical tread patterns are each formed on a portion of a tread band, and each of the portions of tread band is formed from elastomeric compounds having a different modulus of elasticity (measuring the energy retained and recovered during a cyclic deformation), the resulting synergetic interaction between the tread band compound and the tread pattern provides a tire with improved comfort and smoothness level as well as improved handling characteristics in dry road conditions are achieved.
The improvement in handling and comfort was especially marked when the inner portion of the tread band, towards the vehicle, was formed of a compound having a lower modulus of elasticity, and had a tread design optimized for operating on wet roads. Conversely, the outer portion of the tread band, further from the vehicle, was formed of a higher modulus of elasticity compound, and had a tread design optimized for dry road cornering.
One aspect of the invention is a tire having an asymmetrical tread pattern and providing improved comfort and handling characteristics.
The tire has a radial carcass having two opposing edges; sidewalls extending along the edges of the radial carcass; a tread band disposed radially outward of the carcass; and a belt structure interposed between the carcass and the tread band. When the tire is mounted on a rim that is secured to a vehicle, the inner side of the tire is the side nearest to the vehicle centerline, while the outer side is the side furthest from the vehicle centerline during normal operation. The tread band has a first portion made of elastomeric compound having a first modulus of elasticity, and a second portion made of elastomeric compound having a second, different modulus of elasticity. The first portion of the tread band has a first modulus of elasticity at 23xc2x0 C. of between 12 MPa and 16 MPa. The second portion of the tread band has a second modulus of elasticity at 23xc2x0 C. of between 7 MPa and 11 MPa. The first portion also includes a tread pattern that is different from the tread pattern of the second portion.
The two portions of the tread band are circumferential bands that can have a constant width. The first portion is located adjacent to the outer side of the carcass and has a tread pattern optimized for resisting cornering forces when the vehicle is maneuvering, and for providing good handling performance on dry surfaces. The second portion is located adjacent the inner side of the carcass and has a tread pattern optimized for traction in wet and rainy conditions, and for resisting hydroplaning.
The first portion of the tread band has a modulus of elasticity which is greater by at least 1 MPa than the modulus of elasticity of the second portion of the tread band, and more preferably is greater by at least 3 MPa.
In another aspect, the invention is a tread band of a tire for improving the handling and comfort characteristics of the tire. The tread band has a first tread band portion having a first modulus of elasticity and having a first tread pattern a second tread band portion formed of a second elastomeric compound having a second modulus of elasticity, and having a second tread pattern engraved thereon, wherein the first modulus of elasticity is at least 1 MPa greater than the second modulus of elasticity, at 23xc2x0 C.
The first tread band portion is disposed adjacent to the outer side of the tire.
In yet another aspect of the invention, there is provided a method for improving the handling and comfort characteristics of an asymmetrical tire, including the steps of forming a tire with a first portion of tread band having a first modulus of elasticity, forming a second tread band portion having a second modulus of elasticity, forming on the first portion of tread band a first tread pattern which is optimized for dry roads, and forming on the second portion of the tread band a second tread pattern which is optimized for wet roads.
In a further aspect, the invention is a wheel designed to be mounted on a vehicle, and having a rim and a tire. The tire has an asymmetrical tread band with a first portion made of an elastomeric compound having a first modulus of elasticity and a first tread design, and a second portion made of a compound having a different second modulus of elasticity and second tread design. The modulus of elasticity and the tread pattern of said first portion being more optimized for dry roads and the modulus of elasticity and the tread pattern of said second portion being more optimized for wet roads.
It is to be understood that both the foregoing general description and the following detailed description of the invention are exemplary and explanatory, and are intended to provide further explanation of the invention as claimed.